A data center is a location in which various services are provided, such as, data processing, data storage and data exchange. To provide better services, a service provider generally deploys a plurality of data centers (also referred to as sites, and each site usually includes multiple physical devices) at different geographical locations for load balancing and for higher reliability. Furthermore, hardware resources of multiple servers can be virtualized to form a resource pool by using server virtualization techniques. A virtual server may be capable of managing multiple hardware facility platforms, and enabling a virtual machine to move freely from one data center to another. Moving of a virtual machine should be transparent to users. therefore the virtual server should have the same IP address after the move. Therefore it is desirable to provide layer 2 interconnection between data centers in different geographical locations.